The present invention relates generally to fiber optic communication, and more particularly to modulation of light for use with fiber optics in fiber optic communication schemes.
With the advent of dispersion compensating fibers, erbium doped fiber amplifiers, high speed amorphous silicon detectors, and all optical demultiplexing, fiber optic transmission speed is limited principally by the modulation speed of the optical transmitter.
High speed modulators have been invented that take advantage of the properties of superconducting materials. Superconducting materials are in there "superconducting" state if the current density in the material, and the temperature of the material, and the magnetic field around the material are all below certain critical values. The critical current density (J.sub.c), critical temperature (T.sub.c), and the critical magnetic field (H.sub.c) are all dependent on the chemical composition of the material and on the presence or absence of defects and impurities. If any of these quantities rise above the critical values the material leaves its superconducting state and enters its "normal" state. The material has properties similar to a semiconductor in it's normal state and is characterized by a normal-state resistivity. The superconducting state has many of the properties of a theoretically perfect conductor. The electrical resistance is zero and electromagnetic fields are reflected by it. Thus in the superconducting state a superconducting thin film acts like a mirror with 100% reflectivity 1, 2!. In the normal state light is partially transmitted 3!. The bracketed end note reference numbers 1, 2! and all other such end notes and reference numbers cited herein appear at the end of this specification along with the reference notes themselves.
U.S. Pat. No. 5,210,637 which is incorporated herein by reference issued May 11, 1993 to Puzey for "High Speed Light Modulation" discloses a device for the high speed modulation of light wherein a layer of superconducting film is used to modulate the light. U.S. Pat. No. 5,036,042 which is incorporated herein by reference issued Jul. 30, 1991 to Hed for "Switchable Superconducting Mirrors" and discloses a device that can be used for the high speed modulation of light.
FIG. 1 herein illustrates one embodiment of U.S. Pat. No. 5,210,637 as indicated by reference numeral 10. A DC power supply 15 is connected to a light source 13 to provide constant light output. A superconducting film 14 is placed in the path of the optical output and its reflectivity is altered by a modulating circuit 16 which switches the film between its superconductivity and non-superconductive states, as described In the Puzey patent. The altered reflectivity results in optical pulses 20 which are carried away by an optical fiber 25. The superconducting film is kept cool by placing it in a dewar 22 which is cooled by means of a refrigerating device 26.
A key drawback of device 10 and the corresponding device in the Hed Patent is that they are both limited to creating optical pulses in the far infrared range (approximate wavelength of 14 microns). This is because at higher frequencies the photon energy of the light is high enough to break the binding energy of the Cooper electron pairs responsible for the phenomena of superconductivity. In order for the device to work properly the photon energy of the light must be less than the binding energy (or energy gap) of the cooper pairs. This relation is given by the formula below: EQU hv&lt;2.DELTA. {1}
Where h is Planck's constant, v is the frequency of light, and 2.DELTA. is the energy gap of the superconductor. The energy gap of the superconductor can be found from Mattis-Bardeen 4!. EQU 2.DELTA.=8 k T {2}
Where k is Boltzman's constant, and T is the critical temperature of the superconducting material. High critical temperature Thallium compounds have critical temperatures around 128 Kelvin. Plugging this into equations {1} and {2}, the operation of the device is limited to light with a wavelength around 14 microns.
The attenuation of light in silica glass fiber (the most common material for long haul fibers) can be calculated from the formula below. EQU .alpha.=Ae.sup.-a/.lambda. +B/.lambda..sup.4 { 3}
Where .alpha. is the attenuation; A, a, and B are constants that are material dependent. .lambda. is the wavelength. The attenuation of 14 micron light in silica glass fiber is approximately 7.32.times.10.sup.10 dB per km using formula {3} and data from reference 5!. Therefor, applicant has found that the attenuation of 14 micron light in glass fiber is to high to be useful for telecommunication. Modern telecommunication systems are optimized for wavelengths around 1.3 or 1.55 microns and have attenuation around 0.15 dB per km. Unfortunately, light at these wavelengths (i.e. at these higher photon energies) are not compatible with the devices described in the Puzey and Hed Patents. The present invention to be described hereinafter provides a solution to this problem which has remained unsolved since as long ago as December 1988, the filing date of the '042 Hed patent.